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<title>Operator Precedence Problems (The C Preprocessor)</title>

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<h4 class="subsection" id="Operator-Precedence-Problems-1"><span>3.10.2 Operator Precedence Problems<a class="copiable-link" href="#Operator-Precedence-Problems-1"> &para;</a></span></h4>
<a class="index-entry-id" id="index-parentheses-in-macro-bodies"></a>

<p>You may have noticed that in most of the macro definition examples shown
above, each occurrence of a macro argument name had parentheses around
it.  In addition, another pair of parentheses usually surround the
entire macro definition.  Here is why it is best to write macros that
way.
</p>
<p>Suppose you define a macro as follows,
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define ceil_div(x, y) (x + y - 1) / y
</pre></div>

<p>whose purpose is to divide, rounding up.  (One use for this operation is
to compute how many <code class="code">int</code> objects are needed to hold a certain
number of <code class="code">char</code> objects.)  Then suppose it is used as follows:
</p>
<div class="example smallexample">
<pre class="example-preformatted">a = ceil_div (b &amp; c, sizeof (int));
     &rarr; a = (b &amp; c + sizeof (int) - 1) / sizeof (int);
</pre></div>

<p>This does not do what is intended.  The operator-precedence rules of
C make it equivalent to this:
</p>
<div class="example smallexample">
<pre class="example-preformatted">a = (b &amp; (c + sizeof (int) - 1)) / sizeof (int);
</pre></div>

<p>What we want is this:
</p>
<div class="example smallexample">
<pre class="example-preformatted">a = ((b &amp; c) + sizeof (int) - 1)) / sizeof (int);
</pre></div>

<p>Defining the macro as
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define ceil_div(x, y) ((x) + (y) - 1) / (y)
</pre></div>

<p>provides the desired result.
</p>
<p>Unintended grouping can result in another way.  Consider <code class="code">sizeof
ceil_div(1, 2)</code>.  That has the appearance of a C expression that would
compute the size of the type of <code class="code">ceil_div (1, 2)</code>, but in fact it
means something very different.  Here is what it expands to:
</p>
<div class="example smallexample">
<pre class="example-preformatted">sizeof ((1) + (2) - 1) / (2)
</pre></div>

<p>This would take the size of an integer and divide it by two.  The
precedence rules have put the division outside the <code class="code">sizeof</code> when it
was intended to be inside.
</p>
<p>Parentheses around the entire macro definition prevent such problems.
Here, then, is the recommended way to define <code class="code">ceil_div</code>:
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define ceil_div(x, y) (((x) + (y) - 1) / (y))
</pre></div>

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